According to the combination of a radiation source and a detector, the existing container inspection system or equipment can be generally divided into the combination of electron accelerator, including electron linear and electrostatic accelerator, and scintillation detector or gas detector, and the combination of cobalt-60 γ-ray source or cesium-137 γ-ray source and gas detector. The inspection system or equipment is made from the combination of the electron accelerator as the radiation source and the scintillator as the radiation detector. For example, the British Aerospace Company and the Russian Institute of Electrophysics use the combination of linear electron accelerator and cadmium tungstate (CdWO4) scintillation detector (cf. Jigang An. The research and development of container inspection system, ISOTOPE, Vol.8, No.1, 1995, in Chinese); the Heimann Company (Germany) uses the combination of linear electron accelerator and cesium iodide (cf. Radiation Source and Detector Configuration of the HEIMANN Container Inspection System).
The inspection system or equipment is constituted of electron accelerator as the radiation source and gas detector as the radiation detector. For example, the Schlumberger Company (France) uses electrostatic electron accelerator and gas multi-wire proportional chamber, and Tsinghua Tongfang Nuchtech Co., Ltd. (China) uses linear electron and gas pressurized ionization chamber (Chinese patent CN1242519A).
The inspection system or equipment is constituted of cobalt-60 γ-ray source as the radiation source and gas detector as the detector. For example, Beijing Hualixing Sci-Tech Development Co., Ltd. uses cobalt-60 γ-ray source and gas pressurized ionization chamber (Chinese patent CN1242519A).
The inspection system or equipment manufactured from the three combinations mentioned above has the following defects:
The defects when using electron accelerator as the radiation source are as follows: The electron accelerator is complicated in structure, big and heavy, difficult in manufacture, long production cycle, high manufacture cost. The electron accelerator needs special technicians in charge of operation and maintenance. The power consumption is high (several 10 kW) involving high voltage (several 10 kV to several 100 kV). The key component (e.g. klystron) needs regular replacement. The annual operating expense is very high and the probability of accelerator failure is also higher. It needs not only very high expense in repair and maintenance (the annual maintenance expense for container inspection system of accelerator type will be up to 0.8˜1.28 million USD), but also a long shutdown time for maintenance, thus it directly influence the Customs in search for smuggler. The radiation output from electron accelerator is very high, and the thickness of concrete shielding wall is required to be 2˜2.5 m for the main part of protection building. In addition, the field angle subtended by exposure field of high X-ray produced by accelerator is small (˜30°), then it requires increasing the distance between the accelerator target and the detector to be more than 10 m. Therefore, the building area for inspection system of using accelerator as the radiation source is very large and the building expenditure is very high which may reach to 2.5˜4 million USD. Plus the value of the detector, the total cost for the inspection system using electron accelerator will come to 8˜10 million USD.
The defects when using gas detector are as follows: The detection efficiency of the gas detector is much less than that of the scintillation detector, such as cesium iodide or cadmium tungstate. For example, the detection efficiency of multi-wire proportional chamber is about 25%. In a super high pressure gas ionization chamber, the detection efficiency for cobalt-60 γ-ray is only about 13% when the electrode length is 20 cm and xenon gas is filled to 5×106 Pa (50 atm). The gas multi-wire proportional chamber is a gas flow type detector, and the anode wire in the proportional chamber is very thin (several 10 μm) with length of 200˜300 mm, so that it is easy to induce noise signal due to vibration. The high pressure (pressurized) gas ionization chamber is complicated in structure and more difficult in workmanship. The filling (gas) pressure is 1.0×106 Pa ˜1.0×107 Pa (10˜100 atm). When the ionization chamber is filled with gas to a pressure of 5×106 Pa (50 atm), the pressure resistance on the chamber case will reach to 8×106 Pa (80 atm). For this reason, the ionization chamber case wall will be appropriately thick and also shall be strengthened by adding strengthening rib. The total gas leakage rate of the sealed gas must be less than 1×10−9 torr.L/s, otherwise it will directly influence the working life of ionization chamber. The insulation resistance of insulator used in the ionization chamber shall be more than 1×1012 Ohm, the gas leakage rate shall be less than 1.0×10−10 torr.L /s. In addition, the complicated electrode system, and the thick and heavy case for pressure resistance will markedly increase the volume and the weight of the detecting unit. It is more difficult to make the ionization chamber due to extremely severe requirement in tightness and insulation. The gas detector is easily influenced by external interference factors such as temperature, moisture, vibration and electromagnetics.
Therefore, when electron accelerator and gas detector are used in the container inspection system, not only the technology is complicated, the equipment is heavy and price is high, but also the operation and maintenance expense is very high with the result that many customers are turned away. This greatly limit extensive application in radiation imagery and nondestructive inspection technique field, so that the development in using container inspection system in search for smuggler by the customs is very slow.